Dump trucks have long been known in the art, for use in carrying different types of loads. One such use is in the construction industry for carrying dirt or other construction materials in the dump truck bed. A typical dump truck bed includes a tailgate defining a rear wall of the truck bed. The tailgate retains the material within the bed when the tailgate is in a storage or raised position and permits material to be ejected from the bed as the dump truck bed is raised. U.S. Pat. No. 5,456,521, assigned to the assignee of the present invention, discloses an unloading gate for a dump truck wherein the gate is mounted to one or more rails disposed on the bottom surface of the bed. The unloading gate moves from one end of the bed to the other to scrape and clean the bed after unloading or dumping of the material. The unloading gate in this patent is utilized in conjunction with a conventional dump truck bed which lifts at its forward end closest to the tractor cab and utilizes gravity to dump the material from the dump truck bed.
Applicant has had under development a new material carrying vehicle with a new material ejection system as is described in U.S. patent application Ser. No. 09/160,698, filed Sep. 25, 1998, entitled `Hydraulic System For a Work Machine Having an Ejector Cylinder and a Tailgate Cylinder`, assigned to the assignee of the present invention, which is hereby incorporated in its entirety herein. A variation of the above system is disclosed in U.S. patent application Ser. No. 08/936,128, filed Sep. 24, 1997, entitled `Tailgate Latching Mechanism`, assigned to the assignee of the present invention, and hereby incorporated in its entirety herein.
Such a material eject system has a motor, for example, a hydraulic cylinder, that is connected to an ejector blade having a home position at the front end of the bed. To initiate a material eject operation, the motor moves the ejector blade toward the rear of the vehicle, thereby pushing the material in the bed off of the rear edge of the bed. The ejector blade is then retracted or returned to its home position at the front of the bed. The bed normally includes a tailgate which prevents material from exiting the bed while the material is being transported. During the material eject cycle, the tailgate is moved between its closed and open positions in synchronization with the operation of the ejector blade.
A vehicle having a material eject system has several advantages over traditional dump trucks. For example, the vehicle bed is not raised; and its center of gravity is not elevated as the material is being ejected. Therefore, vehicle stability is not adversely affected by the material ejection process. By maintaining a lower center of gravity, a material ejection system is more suitable for controlled spreading of the material during the ejection process. Further, by not raising the bed, the vehicle is not exposed to overhead obstacles such as trees and power lines. In addition, the profile of the ejector blade closely matches the cross-sectional profile of the bed and therefore, substantially all of the material in the bed is scraped out of the bed during the ejection process. Thus, a vehicle material ejection system is generally more efficient and flexible than the traditional elevating material dumping systems.
As previously mentioned, the profile of the ejector blade substantially matches the cross-sectional profile of the bed; and therefore, the ejector blade is very effective at scraping a material from all surfaces of the bed. When ejecting certain stickier materials a problem may arise at the end of the material eject stroke. With no relative motion between the ejector blade and the side walls of the bed, a sticky material may bridge between the rear surface of the ejector blade and the inside surfaces of the side walls of the bed. In the absence of any relative motion in the normal material eject cycle, it is impossible to break that bridge of sticky material. When the ejector blade is moved back with the retract cycle, the material continues to stick to the ejector blade and is carried back into the bed. Thus, there is a need to provide a material eject cycle having the capability of breaking any such material bridge at the end of the material eject stroke to completely empty the bed.